This invention relates to electrical equipment protective devices generally and more particularly, to a circuit breaker, operating under low current conditions, that includes a clapper armature system for tripping the circuit breaker in response to a short circuit condition.
Circuit breakers typically provide protection against persistent overcurrent conditions and against very high currents produced by short circuits. This type of protection is provided in many circuit breakers by a thermal-magnetic trip mechanism having a thermal trip portion and a magnetic trip portion, similar to that shown in FIG. 1. The trip mechanism 10 of FIG. 1 includes a conductor 12 that carries current from a load terminal to the pair of contacts for interrupting current in response to an overcurrent or short circuit condition.
The thermal trip portion 13 of the trip mechanism 10 includes a bimetallic strip 14 having one end 16 attached to the conductor 12. The bimetallic strip is formed of two metals having different coefficients of expansion such that a free end 15 of the bimetallic strip bends or deflects counterclockwise when the temperature exceeds a predetermined temperature. As shown, the bimetallic strip 14 is disposed adjacent and substantially parallel to a portion of the conductor 12. When an overcurrent condition occurs, the conductor generates heat, which in turn increases the temperature of the bimetallic strip. If the temperature of the bimetallic strip exceeds the predetermined set point, the free end 15 of the bimetallic strip deflects to actuate a linkage interconnected to the pair of separable contacts. The linkage then opens the pair of contacts to interrupt the current and thereby, protect the load from the overcurrent condition.
The magnetic trip portion 17 of the trip mechanism 10 includes a clapper 18 having one end 20 pivotally connected to the housing of the circuit breaker and a free end 22 that engages the linkage to open the pair of separable contacts in response to a short circuit condition. As shown in FIG. 1, the clapper is disposed adjacent the bimetallic strip 14. A generally U-shaped yoke 24 is disposed about the conductor 12 and the bimetallic strip. Arms 26 and 28 of the yoke extend proximate the clapper 18. When a short circuit condition occurs, a magnetic field in the yoke is generated proportional to the current passing through the conductor. When the magnetic force attracting the clapper 18 is greater than a predetermined level, the clapper pivots clockwise to engage the yoke 24 and actuate the linkage to open the contacts.
The trip mechanism 10 of FIG. 1 is commonly used to protect loads that operate under high current conditions, but not for low operating current conditions. Generally these thermal-magnetic trip mechanisms 10 are unable to afford protection with electric current in the range of 16 to 60 amperes. Such current level is unable to induce a magnetic field of the intensity required for clapper movement when short current protection is required. Typically, the magnetic trip portion 17 of current trip mechanisms 10 for circuit breakers includes a solenoid that is substantially more sensitive to the low current operating conditions.